为什么 Rust 允许通过空指针调用函数？

我正在尝试 Rust 中的函数指针魔法，最终得到了一个代码片段，我完全没有解释为什么它会编译，甚至没有解释它为什么运行。

fn foo() {
    println!("This is really weird...");
}

fn caller<F>() where F: FnMut() {
    let closure_ptr = 0 as *mut F;
    let closure = unsafe { &mut *closure_ptr };
    closure();
}

fn create<F>(_: F) where F: FnMut() {
    caller::<F>();
}

fn main() {
    create(foo);
    
    create(|| println!("Okay..."));
    
    let val = 42;
    create(|| println!("This will seg fault: {}", val));
}

我无法解释why foo正在通过在中强制转换空指针来调用caller(...)到类型的实例F。我本以为函数只能通过相应的函数指针来调用，但考虑到指针本身为空，显然不可能是这种情况。话虽如此，我似乎显然误解了 Rust 类型系统的一个重要部分。

这个程序实际上根本没有构造函数指针——它总是调用foo和这两个闭包直接地。

每个 Rust 函数，无论是闭包还是fnitem，具有独特的匿名类型。该类型实现了Fn/FnMut/FnOnce特征，视情况而定。 a 的匿名类型fnitem 的大小为零，就像没有捕获的闭包类型一样。

因此，表达式create(foo)实例化create的参数F with foo的类型 - 这不是函数指针类型fn()，但只是一个匿名的、零大小的类型foo。在错误消息中，rustc 调用此类型fn() {foo}， 如你看到的.

Inside create::<fn() {foo}>（使用错误消息中的名称），表达式caller::<F>()将此类型转发到caller而不给它该类型的值。

Finally, in caller::<fn() {foo}> the expression closure() desugars to FnMut::call_mut(closure). Because closure has type &mut F where F is just the zero-sized type fn() {foo}, the 0 value of closure itself is simply never used¹, and the program calls foo directly.

同样的逻辑也适用于闭包|| println!("Okay...")，其中喜欢foo有一个匿名的零大小类型，这次称为类似的东西.

The second closure is not so lucky- its type is not zero-sized, because it must contain a reference to the variable val. This time, FnMut::call_mut(closure) actually needs to dereference closure to do its job. So it crashes².

¹ Constructing a null reference like this is technically undefined behavior, so the compiler makes no promises about this program's overall behavior. However, replacing 0 with some other "address" with the alignment of F avoids that problem for zero-sized types like fn() {foo}, and gives !)

² Again, constructing a null (or dangling) reference is the operation that actually takes the blame here- after that, anything goes. A segfault is just one possibility- a future version of rustc, or the same version when run on a slightly different program, might do something else entirely!